Haunted by accusations of anti-Americanism from the Vietnam era, and under pressure from a growing atmosphere of intolerance towards protesters, one anti-war group has unveiled a huge billboard with the message "Peace is Patriotic" against the image of an unfurling Stars and Stripes.

One of the nation's largest groups, Win Without War, is encouraging the two million people on its email list to send supportive letters to soldiers.

"We know that this is what the pro-war folks try to slam us on," says Medea Benjamin, the founding director of the San Francisco-based advocacy group, Global Exchange.

Ms Benjamin, a spokeswoman for the 30,000-strong group, which is raising money for more patriotic billboards, believes the strategy of questioning the patriotism of protesters is already silencing potential dissenters.

"It is working with the progressive wing of the Democratic party. We want to reclaim the right to portray the flag. For all those who want to show their sense of patriotism and oppose the war, we want to create a space for that."

It is just one sign of both a blatant and subtle shift in strategy among the loose confederation of different organisations that make up the peace movement, now that war has started. Many have been keen to distinguish between their campaign against the politicians who have launched the war and their support for the soldiers who are fighting it.

"One of the ways in which our message has changed since the war started is that we are making it clear that our opposition to the war doesn't mean that we're opposed to the troops," says Leslie Cagan, a spokeswoman for United for Peace and Justice, the umbrella group that organised the largest demonstrations in recent weeks. "We want to support them by bringing them home," she says.

Some have also shifted their goals of civil disobedience from disrupting everyday life with "die-ins" on busy streets to drawing attention to the collusion of corporate, federal or media institutions in the war effort. The change is partly the product of a more media-savvy movement, which is now armed with its own public relations advisers to help sell their message.

But the move to wrap themselves in the flag is a defensive one, prompted by history and by a more belligerent mood in the country as a whole.

"I do a lot of radio shows and you can feel the difference already," says Ms Benjamin.

"A few months ago people were a lot more prepared to listen to the arguments and not just say "Go back to Iraq".

"This was exactly what happened during the Vietnam war days when we were labelled as anti-American, and we don't want that to be repeated," she says.